This invention relates to position sensors and particularly to angular or rotary position sensors.
Position measurements, including both linear and angular measurements, are widely implemented in industrial automation control. In particular, the automotive industry is using more and more linear/angular position sensors for closing various control loops. For example, sensors are used in steer-by-wire systems to determine the angular position of the steering column; sensors are used to determine the angular position of the throttle in engine control modules; sensors are used to determine the brake pedal position and/or the brake master cylinder position in brake-by-wire systems; and sensors are used in vehicle smart suspension systems.
Known technologies that can be used to determine angular position include contact measurement, such as a resistance stripe, or non-contact measurement effects, based on inductance, capacitance, optical, or magnetic field. Sensors based upon a capacitive effect have been found to be particularly desirable in many automotive applications. Whereas some known capacitive position sensors are generally effective to provide an accurate indication of angular position in a non-contact environment, they tend to be rather complex and rather expensive and therefore not always amenable to the high volume and low cost requirements of automotive applications.
This invention is directed to the provision of an improved angular position sensor. More particularly, this invention is directed to the provision of an improved capacitive angular position sensor especially suitable for various automotive applications.
The sensor of the invention is intended for use in sensing the angular position of a rotatable body such, for example, as a steering column of a motor vehicle.
According to the invention, the sensor includes a stationary transmitter capacitor plate defining a transmitter surface area, the transmitter surface area including at least one transmitter electrode and a stationary receiver capacitor plate defining a receiver surface area generally corresponding in size to the transmitter surface area, the receiver surface area including at least a first receiver electrode and a second receiver electrode, the electrodes of the respective capacitor plates facing each other. Positioned in an air gap between the capacitor plates is a rotor formed of a dielectric material adapted to be fixedly secured to the rotatable body so as to rotate with the rotatable body. The rotor defines a rotor area larger than the transmitter surface area and the receiver surface area and is sized so that, in response to angular movement of the rotatable body, the rotor varies a capacitance between each transmitter electrode and an opposed receiver electrode. The sensor includes means for measuring the charge induced on the receiver electrodes whereby the charges indicate the angular position of the rotatable body.
The sensor can include an alternating current source for supplying an excitation signal to at least the first transmitter electrode. Preferably, the sensor includes means for comparing a first charge induced on the first receiver electrode to the second charge induced on the second receiver electrode to determine the angular position.
In a preferred embodiment of the invention, the transmitter capacitor plate is generally circular with an aperture adapted to receive a shaft of the rotatable body and includes a first transmitter electrode and a second transmitter electrode, the first and second transmitter electrodes equally-sized and located about an outside edge of the transmitter capacitor plate. This embodiment can include means for supplying a first alternating current (AC) excitation signal to the first transmitter electrode and for supplying a second AC excitation signal to the second transmitter electrode wherein the first and second AC excitation signals are the same amplitude but with 180 degrees out of phase from each other. These AC excitation signals are preferred to be square waveform signals.
In another embodiment of the invention, the receiver capacitor plate is generally circular with an aperture adapted to receive a shaft of the rotatable body and includes four equal-sized receiver electrodes located about an outside edge of the receiver capacitor plate, each of two diametrically opposed electrodes forming a receiver electrode pair. Preferably, then, the rotor has a semi-circular outside edge larger in size to a portion of the outside edge of the receiver capacitor plate, the size of the portion equivalent to a size of two receiver electrodes with a larger radius. Thus, the high dielectric constant of the rotor as compared to the air gap will result in changing capacitance between the transmitter electrodes and at least one of the receiver electrode pairs.
Yet another embodiment of the invention is seen where each of the capacitor plates is circular with aligned central apertures through which a shaft of the rotatable body can rotate, and the rotor has a semicircular configuration and is adapted to be fixedly secured to the shaft at a center of the semicircular circumference of the rotor.
In a particularly preferred aspect of the invention used to measure 360 degrees of rotation of the rotatable body, the transmitter capacitor plate is generally circular with an aperture adapted to receive a shaft of the rotatable body and includes a first transmitter electrode and a second transmitter electrode, the two electrodes being equally-sized and generally semi-circular. Similarly, the receiver capacitor plate is generally circular with an aperture adapted to receive the shaft and includes four equally-sized receiver electrodes located about an outside edge of the receiver capacitor plate, each of two diametrically opposed electrodes being connected to form a first receiver electrode pair and a second receiver electrode pair. The rotor has a semi-circular shape and is adapted to be fixedly secured to the shaft at a center of the semicircular circumference of the rotor. The rotor with a larger radius is sized so that, in response to rotation of the shaft, the rotor varies the capacitance between the first transmitter electrode and a first pair of adjacent receiver electrodes and the capacitance between the second transmitter electrode and a second pair of adjacent receiver electrodes. Finally, a charge to voltage measuring means of the sensor converts a first charge induced on the first receiver electrode pair and converts a second charge induced on the second receiver electrode pair whereby the first and second converted voltages indicate the angular position of the rotatable body.
This embodiment can include means for supplying a first AC excitation signal to the first transmitter electrode and for supplying a second AC excitation signal to the second transmitter electrode wherein the first and second AC excitation signals are 180 degrees out of phase from each other. This supply means can include a square wave generator with a frequency in a preferred range of 20 to 100 kHz supplying the first AC excitation signal and an analog inverter receiving the first AC excitation signal and producing the second AC excitation signal.
The voltage measuring means can include a current-to-voltage converter for receiving a current flow from one receiver electrode of a receiver electrode pair to the other receiver electrode of the receiver electrode pair and producing an AC voltage representing a charge induced on the receiver electrode pair. Then, the sensor can include means for converting the AC voltage to a direct current (DC) voltage.
The means for converting the AC voltage can include an integrating capacitor for receiving the AC voltage and converting the AC voltage to a DC voltage. In an embodiment including this feature, the sensor can also include means for connecting the integrating capacitor to receive the AC voltage only during a positive half of the first AC excitation signal.
In order to minimize temperature effects by having separate voltage measuring channels, only one voltage measuring means is preferred to measure the voltages of each receiver electrode pair. Thus, the sensor preferably includes a receiver pair select switch for selectively enabling a current flow from either the first receiver electrode pair or the second receiver electrode pair, depending upon the position of the switch. In order to sample both, the sensor may include means for controlling the receiver pair select switch.
In an embodiment including the integrating capacitor, the sensor can also compare a first DC voltage at the integrating capacitor resulting from a current flow from one receiver electrode of the first receiver electrode pair to the other receiver electrode of the first receiver electrode pair to known voltages corresponding to angular positions of the rotatable shaft. This sensor also compares a second DC voltage at the integrating capacitor resulting from a current flow from one receiver electrode of the second receiver electrode pair to the other receiver electrode of the second receiver electrode pair to the known voltages. The actual angular position is the result of the comparisons. This can be done using a look up table in an integral microcontroller or in the engine microcontroller.
Preferably, the receiver capacitor plate includes a guard trace on the receiver surface area, the guard trace adjacent an outside edge of the receiver capacitor plate and located so as to prevent the interaction of adjacent electric fields. Of course, the transmitter capacitor plate can include such a guard trace, which is particularly desirable when the plate includes two transmitter electrodes.
Other applications of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.